1. Field of the Invention
The present invention relates to a method for manufacturing an optical memory element, such as an optical memory disc, also to a photo-mask used in the disc manufacturing step, and further to a method for making the photo-mask.
2. Description of the Prior Art
Recently, the need for the optical memory discs has increased remarkably, because of their high density and high capacity memory ability. Optical memory discs can be divided into three types: a read-only type which can only reproduce the recorded data; a read/add type which can reproduce the recorded data and also add further data, but can not erase the original or the added data; and a read/write type which can reproduce, erase and record data freely.
As shown in FIG. 1, the discs of the read/add type and the read/write type are previously provided with guide tracks 111 for guiding an area to be recorded, and address indications 112 defined by a number of microscopic pits in the form of minute indentations spaced along the spiral or concentric guide tracks for indicating the addresses of the guide tracks. In the case where each guide track is divided into a number of sectors, sector indications are further provided in the form of indentations.
A prior art method for manufacturing the optical memory disc is disclosed, for example, in Japanese patent laid-open publication No. 60-195751 (corresponding to the above-mentioned U.S. patent application Ser. No. 017,456, filed Feb. 24, 1987, Kenji OHTA et al., entitled "Method of Manufacturing Optical Memory Element") assigned to the same assignee as the present application. The prior art method is briefly described hereinbelow in connection with FIGS. 2a-2e.
First, a glass disc 100 having no track or no indentation is deposited with a photoresist film 101 using a spinner, or the like (FIG. 2a). Then, a photo-mask M defined by a transparent plate 102 and opaque film 105 having a predetermined pattern and made of, e.g., Cr or Ta is placed on the photoresist film 101 and, thereafter, the photoresist film is exposed by light, such as ultraviolet light UV (FIG. 2b). Then, the photoresist is developed in a certain agent so as to remove the photoresist which has been exposed by the light and for the non-exposed areas to remain (FIG. 2c). Thereafter, a reactive ion etching is carried out in a chamber filled with CF.sub.4 or CHF.sub.3 gas, or instead, a wet etching is carried out in a pool filled with HF liquid, so as to form guide tracks 111 and indentations 112 in the glass disc 100 (FIG. 2d). Finally, the remaining photoresist is removed by solvent such as acetone, or by sputtering in O.sub.2 plasma so as to produce a glass disc formed with guide tracks and indentations (FIG. 2e).
In the prior art method as described above, the photo-mask M is formed by the following steps.
As shown in FIG. 3, the transparent plate 102 in a shape of disc is deposited with an opaque film 105 and further with a photoresist film 108. Then, while rotating the disc about its center, an Ar laser beam 110 converged by a lens system 109 impinges on the photoresist film 108. A continuous laser beam is produced when tracing the guide tracks and an intermittent laser beam is produced when tracing the places where the indentations are to be formed. Thereafter, the photoresist film is partly removed in a pattern corresponding to the laser beam impingement. Then, by a certain agent, opaque film 105 is etched according to the pattern, and the remaining photoresist is removed, thereby producing the photo-mask M.
According to the prior art manufacturing steps shown in FIGS. 2a-2e, the guide tracks and the indentations will have the same depth. However, according to a recent technical development, it has been found that it is preferable to make the depth D1 at guide tracks smaller than the depth D2 at indentations, as depicted in FIG. 1, so as to improve the signal recording quality. For example, when the tracks are traced by a light beam having a wavelength of .lambda. (.lambda.=7800 .ANG. or 8300 .ANG.) so as to read/write the disc through diffraction differential system, it is preferable to make the depth D1 approximately equal to .lambda./8n (n is a refractive index of the substrate 100), and the depth D2 approximately equal to .lambda./4n.
Thus, problem is that the prior art manufacturing steps as shown in FIGS. 2a-2e will not result in the optical memory disc shown in FIG. 1.